1. Field of the Invention
This invention pertains to environmental protection, and more particularly to apparatus for collecting and disposing of industrial coatings.
2. Description of the Prior Art
Millions of three-piece steel cans are manufactured daily. The process of manufacturing the bodies of the steel cans involves many carefully controlled steps. Generally, a flat sheet of steel is coated on both sides with a thin layer of an organic compound that protects the can from the product the can is to hold. Narrow margins on both faces along two opposed edges of the sheet are left uncoated. The sheet is rolled such that the uncoated edges abut to form a thin walled tube with a longitudinal axis. The tubes are propelled sequentially along their longitudinal axes in a downstream direction past a welding station. At the welding station, in-line continuous welding systems operate to weld together the abutting edges to form a stable thin walled tube that serves as the can body.
From the welding station, the can bodies continue downstream to a liquid stripe application or coating station. At the coating station, stationary nozzles spray the continuously moving can bodies along their welded seams with the proper organic compound such that the entire inner and/or outer surfaces of the can bodies are properly coated. The coating materials typically are volatile organic compounds such as lacquers, enamels, and vinyls, and they are composed of known solids and solvents. From the coating station, the cans move downstream for further processing.
Coating the welded seams of the can bodies presents several difficult problems. The coating material must be accurately directed so as to strike and coat the welded seams while at the same time coating as little of the adjacent can areas as possible. To avoid spraying excessive material, the coating equipment must operate in a carefully controlled manner so that the coating material is sprayed only when a can is present at the coating station during its continuous downstream motion. Two paramount requirements are to minimize overspray of the coating material and to prevent any overspray from continuing downstream or from entering the atmosphere.
To collect coating material overspray and prevent it from entering the atmosphere, it is known to provide the liquid stripe application or coating stations of seamed can bodymakers with vacuum operated exhaust systems. The exhaust systems collect the overspray and direct it to a filter that traps the coating material solids for subsequent disposal. The solvents of the coating material overspray pass through the filter to be burned or otherwise properly disposed of.
FIGS. 1 and 2 show simplified side and front views, respectfully, of a prior exhaust system 1 for a seamed can bodymaker. The can bodies 3 travel continuously at high speeds in the downstream direction of arrow 5. Stationary nozzles schematically represented by reference numeral 7 spray coating material on the external and internal surfaces of the can bodies 3 along the welded seams 8 thereof as the can bodies travel past the nozzles.
The prior exhaust system 1 includes a hood 9 with an open slot 11 that is a short distance above the spray nozzles 7. The hood 9 has an arcuate bend of approximately 90 degrees. Depending upon the specific application, the length of the slot 11 may range between approximately nine and 18 inches and have approximately a two inch width. The hood connects via a throat 13 with an exhaust passage 15. The areas of the hood inlet slot 11 and outlet throat 13 are generally equal. The exhaust passage 15 opens into a filter box 17.
To draw overspray from the nozzles 7 into the exhaust system 1, a vacuum is created in the filter box 17, exhaust passage 15, and hood 9 by a blower, not shown, that is connected to a filter box stack 19. The overspray thus flows through the hood to the filter box, where the solids in the coating material overspray are separated. The remaining solvents are drawn out through the stack 19 for appropriate processing.
During normal operation, some overspray from the coating material coagulates into a gel like substance 20 on the hood concave inner surface 18. The coagulant 20 tends to drip back through the hood slot 11 and onto the can bodies 3. Further, as the solids coagulate on the hood surface 18, the area in the hood 9 through which the overspray material must flow decreases. Consequently, the pressure drop required to maintain adequate overspray flow through the hood and the rest of the exhaust system 1 increases, thereby resulting in increased power consumption by the blower. To maintain proper operation, the substance 20 must be removed from the surface 18 at regular intervals. For example, with some spray materials, the coagulant must be cleaned from the hood surface 18 after approximately eight hours of system operation.
The prior exhaust system 1 functions adequately, and numerous installations have been in successful operation for many years. Despite the fact that the hood 9 must be removed from the rest of the exhaust system for cleaning the overspray solids 20, the frequency of cleaning is tolerable. However, increasingly stringent environmental considerations have made the problems associated with coating material overspray collection much more difficult to solve. Particularly, whereas formerly high solvent coating materials were acceptable, recent regulations dictate that high solid content coating materials now be used. Unfortunately, the prior exhaust system 1 does not work as well with high solid coating materials as with high solvent materials. High solid coating materials tend to coagulate at much faster rates on the hood surface 18 than high solvent materials. As a result, more frequent cleaning of the hood surface 18 is necessary. In some installations, the surface 18 must be cleaned approximately two times oftener with the new high solid coating materials than with previous coating materials. That increase in the frequency of cleaning is unacceptable.
Thus, a need exists for an overspray exhaust system that is capable of handling high solid content coating materials.